Until the wife is able to immerse, a husband and wife who had a chupat niddah are not permitted to be in yichud (halachically defined seclusion) with each other, much like a couple who are dating.

Whether or not being the only passengers in a car violates the prohibition of yichud is a matter of halachic debate. A couple may rely on the more lenient halachic decisors, who permit it so long as the car trip is on well-traveled roads during hours when other cars tend to be on the road.

Please write back with any further questions.